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At the summit of a dense forested plateau in Venezuela is the world's largest sandstone sinkhole, Sima Humboldt, which drops 1,000 feet (314 m) . Its neighbor, Sima Martel, is another impressive sinkhole. At the bottom of Sima Humboldt is a completely isolated forest. Sinkhole openings where mini-jungles have grown are known as dolines, and they are filled with life, which has been buffered from the outside world by the cave. - Credit: Dreamstime